The baby, born in rural Mississippi, was treated aggressively with antiretroviral drugs starting around 30 hours after birth, something that is not usually done. If further study shows this works in other babies, it will almost certainly change the way newborns of infected mothers are treated all over the world. The United Nations estimates that 330,000 babies were newly infected in 2011, the most recent year for which there is data, and that more than 3 million children globally are living with H.I.V.

If the report is confirmed, the child born in Mississippi would be only the second well-documented case of a cure in the world, giving a boost to research aimed at a cure, something that only a few years ago was thought to be virtually impossible.

The first person cured was Timothy Brown, known as the “Berlin patient,’’ a middle-aged man with leukemia who received a bone-marrow transplant from a donor genetically resistant to H.I.V. infection.

“For pediatrics, this is our Timothy Brown,’’ said Dr. Deborah Persaud, associate professor at the Johns Hopkins Children’s Center and lead author of the report on the baby. “It’s proof of principle that we can cure H.I.V. infection if we can replicate this case.’’

Dr. Persaud and other researchers spoke in advance of a presentation of the findings on Monday at the Conference on Retroviruses and Opportunistic Infections in Atlanta.

Some outside experts, who have not yet heard all the details, said they needed convincing that the baby had truly been infected. If not, this would be a case of prevention, something already done for babies born to infected mothers.

“The one uncertainty is really definitive evidence that the child was indeed infected,” said Dr. Daniel R. Kuritzkes, chief of infectious diseases at Brigham and Women’s Hospital.

“The CDC estimates that 12,200 young men and women aged 13 to 24 became infected with HIV in 2010. And by far most of them were boys and men. Nearly three-quarters were boys and men having sex with other men. And more than half of the newly infected youths were African American.

The report “really provides shocking data on the higher rate of risky behavior and the lower rate of condom use” among young men, CDC Director Dr. Thomas Frieden told reporters on a conference call.

The report finds that just 13 percent of high school students have been tested for the virus and just 35 percent of 18 to 24 year olds have been.

“Young gay and bisexual men report much higher levels of risky sexual behavior than their heterosexual peers,” Frieden said. They are more likely to have multiple sex partners, use drugs and alcohol before sex – which makes them in turn more likely to skip using condoms and tsake on other risks, too.

A separate CDC survey of high school kids and young adults found that young gay and bisexual men were more likely to report having had sex with four or more partners. They were also more likely to have injected drugs – another risk factor for AIDS, although most are infected sexually.

The answer, the CDC says, is a combination of testing, treatment and good sex education in schools.”

CDC data presented on Tuesday show just 47 percent of high school students have ever had sex, down from 54 percent in 1991 and holding steady since about 2001. Much progress has been seen among black students: in 1991, 82 percent of black high school students had started having sex but this plummeted to 60 percent by 2011. Just 15 percent of all students have had more four or more sex partners, down from 19 percent in 1991.

And 60 percent of those who are sexually active used a condom, which can protect against pregnancy and sexually transmitted diseases including the human immunodeficiency virus that causes AIDS….

The CDC’s Dr. Kevin Fenton says it’s the frank talk about sex that works. “The more comprehensive an education you provide, the better,” Fenton said in an interview. But he noted there is variation across the country, with some school districts choosing abstinence-only education while others offer a full curriculum that includes discussion of lesbian gay and transgender themes as well as how to respect one another in a relationship.

Budget cuts aren’t helping. “Data show that fewer schools provide the comprehensive HIV education needed to ensure that this trajectory continues,” Fenton said. Another barrier: socially conservative movements that reject sex education. Fenton is diplomatic when he is asked about school districts and parents who fear that sex education teaches poor morals.

“Part of what we are committed to doing is to provide evidence,” he said. “We try to make our recommendations on the best available evidence.” Studies show that a comprehensive sex education program can influence sexual behavior more than a limited approach.

The study, published on Tuesday in The Lancet, a British science journal, comes ahead of a major family planning conference in London organized by the British government and the Bill and Melinda Gates Foundation that is an attempt to refocus attention on the issue. It has faded from the international agenda in recent years, overshadowed by efforts to combat AIDS and other infectious diseases, as well as by ideological battles.

The proportion of international population assistance funds that went to family planning fell to just 6 percent in 2008, down from 55 percent in 1995, while spending on H.I.V./AIDS represented 74 percent of the total in 2008, up from just 9 percent in 1995, according to Rachel Nugent, a professor of global health at the University of Washington, who cited figures from the United Nations Population Fund.

But population growth has continued to surge, with the United Nations estimating last year that the world’s population, long expected to stabilize, will instead keep growing. Population experts warn that developing countries, particularly those in sub-Saharan Africa, where fertility continues to be high and shortages of food and water are worsening, will face deteriorating conditions if family sizes do not shrink.

Maternal deaths fell to about 287,000 in 2010, the report said. The decline is attributable to increases in contraception and in antiretroviral drugs for mothers with AIDS, and to greater numbers of births attended by nurses, doctors or midwives with medical training.

The agencies said the deaths had fallen by 47 percent from the United Nations’ 1990 estimate of 543,000, but the organization has been revising its historical estimates in response to skeptical research by a rival group of epidemiologists at the University of Washington.